Serial-line-scan-encoded multi-color fluorescence microscopy and imaging flow cytometry

ABSTRACT

A system for performing high-speed, high-resolution imaging cytometry includes a scanning region that is illuminated by light including at least first and second wavelength bands. The system also includes a cell transport mechanism that transports a cell through the scanning region such that the cell is illuminated. The system further includes a set of at least one linear light sensor, and an optical system that selectively directs light emitted from the cell to two portions of the linear light sensor set such that emitted light in a third wavelength band is primarily directed to a first portion of the linear light sensor set, and emitted light in a fourth wavelength band is primarily directed to a second portion of the linear light sensor set. The system repeatedly takes readings of light falling on the linear light sensor set while the cell is transported through the scanning region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/726,628 filed Mar. 18, 2010 and entitled “Serial-Line-Scan-EncodedMulti-Color Fluorescence Microscopy and Imaging Flow Cytometry,” whichclaims priority to U.S. Provisional Patent Application No. 61/162,072,filed Mar. 20, 2009, and U.S. Provisional Patent Application No.61/232,113, filed Aug. 7, 2009, the entire disclosures of which arehereby incorporated by reference for all purposes as if fully set forthherein.

BACKGROUND OF THE INVENTION

Cytometry is a technical specialty concerned with the counting andcharacterization of biological cells. FIG. 1 shows a simplified diagramof one technique known as flow cytometry. In a basic form of flowcytometry, cells 101 are suspended in a fluid and entrained single-filein a narrow transparent tube 102. The entrainment can be accomplished byany of several methods, including hydrodynamic focusing. A light source103 illuminates each cell 101 as it passes a measurement location 104.Light source 103 may be, for example, a laser. Light from light source103 is scattered by the cell 101 being measured. Some light 105 isscattered generally in the same direction as it traveled to reach thecell 101. Light 105 is sometimes called “forward scatter”, and may becollected by a forward sensor 106. Some light may be scattered in otherdirections as well. This light may be called “side scatter”, and some ofthe side scattered light 107 may be collected by one or more othersensors 108. Output signals from sensors 106 and 108 are sent to acomputer 109, which may store and analyze the signals. By analyzing theamount and distribution of the scattered light, it is possible todiscern information about each cell, for example its size and someinformation about its internal structure.

Flow cytometry may measure the scattered light directly, or may make useof fluorescence. In fluorescence cytometry, the cells may be marked withone or more fluorophores, which are excited by light from source 103 toproduce light by fluorescence. The nature of the emitted light mayreveal additional information about the cells.

The technique shown in FIG. 1 relies entirely on measurements ofscattered light to infer information about the cell structure, but doesnot produce an image of any particular cell. In another technique,called “image cytometry”, an image of an individual cell may be recordedby a camera or microscope.

BRIEF SUMMARY OF THE INVENTION

According to one aspect, a system for performing cytometry comprises ascanning region that is illuminated by light including at least firstand second wavelength bands. The system also comprises a cell transportmechanism transporting a cell through the scanning region such that thecell is illuminated, and a set comprising at least one linear lightsensor. The system further comprises an optical system that selectivelydirects light emitted from the cell to two portions of the linear lightsensor set such that emitted light in a third wavelength band isprimarily directed to a first portion of the linear light sensor set,and emitted light in a fourth wavelength band is primarily directed to asecond portion of the linear light sensor set. The system repeatedlytakes readings of light falling on the linear light sensor set while thecell is transported through the scanning region. In some embodiments,the set comprises at least two linear light sensors. The emitted lightmay be emitted as a result of fluorescence. In some embodiments, thesystem further comprises a slit aperture proximate the linear lightsensor set, such that the system performs semi-confocal imaging. In someembodiments, the set comprises at least two linear light sensors, andimages gathered by the individual linear light sensors in the set arecombined to form an image with improved signal-to-noise characteristicsas compared with an image gathered by a single linear light sensor inthe set. In some embodiments, the images are combined by digitallycombining pixel values from the respective images corresponding tosubstantially the same respective locations on the cell. In someembodiments, the images are combined by time delay integration.

According to another aspect, a system for producing an oblongillumination field comprises a laser that produces a beam, and anillumination lens that receives the beam and causes the beam to convergeor diverge in only a first axis. The system further comprises anobjective lens that is part of an infinity-corrected optical system, theobjective lens receiving the beam after the illumination lens andconverging the beam in a second axis orthogonal to the first. In someembodiments, the illumination lens is a cylindrical lens. In someembodiments, the illumination lens has at least one curved surfacedefined by a circular cylinder. In some embodiments, the system furthercomprises a wavelength-selective mirror between the illumination lensand the objective lens. In some embodiments, the objective lens isspaced from the illumination lens by a distance less than the focallength of the illumination lens. In some embodiments, the objective lensis spaced from the illumination lens by a distance greater than thefocal length of the illumination lens. In some embodiments, the beam isdiverging in the first axis as it leaves the objective lens. In someembodiments, the beam is converging in the first axis as it leaves theobjective lens.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified diagram of a technique known as flowcytometry.

FIG. 2 shows a simplified conceptual diagram of a high-speed,high-resolution line scan image cytometry system in accordance with anembodiment.

FIGS. 3A-3C illustrate an image forming process.

FIG. 3D illustrates an example reconstructed image.

FIG. 4 shows an orthogonal view of a system in accordance with anotherembodiment of the invention.

FIG. 5 illustrates an orthogonal view of a system in accordance withanother embodiment of the invention.

FIG. 6 illustrates an orthogonal view of a system in accordance withanother embodiment of the invention.

FIG. 7 illustrates an orthogonal view of a system in accordance withstill another embodiment of the invention.

FIG. 8 illustrates an orthogonal view of a system in accordance withanother embodiment.

FIGS. 9A-9C illustrate embodiments of a system for producing an oblongillumination field.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 shows a simplified conceptual diagram of a high-speed,high-resolution line scan image cytometry system 200 in accordance withan embodiment. The system of FIG. 2 is a flow cytometry system, althoughone of skill in the art will recognize that embodiments of the inventionmay be utilized in other kinds of cytometry as well.

Cells 101 are entrained in fluid to progress through tube 102 in singlefile. The system may be used to characterize cells of many differentkinds, but in a typical application, cells 101 may be, for example,about 10 to 20 micrometers across, and may progress through tube 102 ata speed of, for example, 10 millimeters per second. A light source 201provides field of light 203 onto tube 102. Light source 201 may be alaser, a light-emitting diode, an incandescent light source, afluorescent light source or another kind of light source. Light source201 may produce substantially monochromatic light, broad spectrum light,or light containing two or more narrow bands of wavelengths. Optionallight shaping element 202 may include various lenses, prisms,reflectors, or other optical components to concentrate light from lightsource 201 into oblong or slit-shaped field 203, through which cells 101are transported. Because, as is described below, only a narrow lineimage will be scanned, only a narrow field need be illuminated, incontrast to traditional epi-illumination in which the entire objectivefield is illuminated. The concentration provided by light shapingelement 202 can increase the effective illumination level by as much astwo to six orders of magnitude as compared with normal, symmetricepi-illumination.

Some light from source 201 is transmitted through or scattered by one ofcells 101, at least a portion of which is within field 203. Some of thelight is redirected by one or more lenses 204 onto a linear sensor 205.Linear sensor 205 may be, for example a charge-coupled device (CCD)sensor, a complementary metal oxide semiconductor (CMOS) sensor, oranother kind of sensor having a plurality of light-sensitive sitesarranged in a row. Lens 204 and sensor 205 may be, for example parts ofa line scan camera such as a Basler Sprint line scan CMOS cameraavailable from Basler AG of Ahrensburg, Germany. The individual sensorsites are sometimes called “pixels”. The corresponding sites at the scanline sensed by the sensor pixels are also sometimes called pixels.Sensor 205 may comprise, for example, one or more rows of pixels, eachrow containing 512, 1024, 2048, or another appropriate number of pixels.The intensity of light falling on the row of pixels may be read byclearing the pixel array, allowing charge to accumulate in the pixelsites for a predetermined exposure time, and then converting theaccumulated charge amounts to numerical values representing the lightintensities. This process is performed repeatedly as the cells pass thescan area. In one example embodiment, the system may take a reading(“scan a line”) every 20 microseconds, or at a scan rate of 50 kHz.Using a cell transport speed of 10 millimeters per second and a scanrate of 50 kHz results in an imaging pixel size of 200 nm. Othertransport speeds and scan rates are possible, and may result in otherimaging pixel sizes. The resulting array of measurements can bereassembled into an approximate image of a cell.

FIGS. 3A-3C illustrate the image forming process. In FIG. 3A, a scanline 301 includes pixels a, b, c, d, and e. A cell 101 is transportedpast scan line 301, as shown in FIG. 3B, which shows scan line 301superimposed on cell 101 at consecutive sample times T₁-T₇. (While FIG.3B shows the cell traversing exactly one pixel per sample time, this isnot a requirement, and in fact will only occur for certain combinationsof cell travel speed, sample rate, and pixel size. In practice,consecutive scanned lines may overlap on the cell being imaged, or theremay be a gap between areas of the cell read by consecutive scan lines.)The light levels read by pixels a, b, c, d, and e are affected by thestructure of cell 101. For example, when no cell crosses scan line 301,relatively high light levels are registered. When a relativelytransparent part of cell 101 crosses a pixel, the light level registeredby that pixel is somewhat reduced. When the nucleus of cell 101 iswithin a pixel, the light level registered at that pixel is may besignificantly reduced. FIG. 3C shows traces of the light levels (on anarbitrary scale ranging from 0 to 1) registered at pixels a, b, and c asa function of time. FIG. 3D shows a reconstructed image, formed bystacking together data scanned during several consecutive line scans,and representing each numerical light reading by a printed gray level.While FIG. 3D is constructed using only a few pixels sampled at a fewtimes and therefore shows a relatively crude depiction of cell 101, inpractice a system according to an embodiment of the invention may scanmore or fewer lines during the passage of each cell, and each line maycontain more or fewer pixels than shown. In one embodiment the systemmay scan approximately 50 lines during the passage of each cell, andeach line may contain approximately 50 pixels. The exact number of linesscanned and pixels affected for each cell will depend on the size of thecells, the line scan frequency, the speed at which cells flow past thescan line, and the particular sensor and optical components used.

The theoretical resolution of the system depends mainly on the qualityof the objective lens. The practical scanning resolution of the systemalso depends on the scan rate, the speed of transport of the cells pastthe scan line, and the particular sensor and optical system used. Thepixel resolution in the Y direction is determined by the imaging system,including the particular lens and sensor used. Pixel resolution in the Xdirection is equal to v·dt, where v is the sample delivery speed and dtis the camera's exposure time. Preferably, v is a known parameter,either pre-determined before a particular flow experiment or measuredduring the course of a cell's passage through the system. Ideally, acell being scanned should be rotation-free and jittering-free during itspassage of the scan line.

The operation of the system of FIG. 2 is described above in the contextof direct light imaging, where scattered light from source 201 ismeasured by sensor 205. A system operating on the same principles couldbe used to perform fluorescence imaging as well, and in fact, the systemmay be especially helpful in fluorescence imaging. In that case, lightfrom source 201 would excite fluorescence in the cell 101 beingmeasured, and resulting emitted light would be collected and measured bysensor 205. The emitted light will generally be at a longer wavelengththan the excitation light from source 201. In fluorescence imaging, itmay be desirable to shield sensor 205 from receiving light from source201, using various filters or geometric arrangements of components, sothat the source light does not overwhelm or interfere with themeasurement of the light emitted by fluorescence. Typically, the lightemitted by fluorescence will be less intense than the source light, andlonger exposure times, stronger illumination, or more sensitive sensorsmay be required for fluorescence imaging than for direct imaging. Also,the shape of the temporal signal changes shown in FIG. 3C may bedifferent in fluorescent imaging than in direct imaging. In directimaging, additional structure in a cell tends to result in less lightbeing received by the corresponding pixel of sensor 205. In fluorescenceimaging, additional structure may carry additional fluorophores, and mayresult in more light reaching the corresponding sensor pixel, ascompared with a pixel corresponding to a cell portion with littlestructure.

FIG. 4 shows an orthogonal view of a system 400 in accordance withanother embodiment of the invention. The embodiment of FIG. 4 may beespecially suited to single-color fluorescence imaging cytometry. In theembodiment of FIG. 4, a light source 401 emits light. Light source 401may be a laser, a light-emitting diode, an incandescent light source, afluorescent light source or another kind of light source. Light source401 may produce substantially monochromatic light, broad spectrum light,or light containing two or more narrow bands of wavelengths. In oneexample embodiment, light source 401 is a laser that emits light at anominal wavelength of 488 nm. An excitation filter 402 may be utilizedto further narrow the band of wavelengths of light utilized by thesystem, especially if light source 401 is a broad spectrum light, orotherwise produces wavelengths that are undesirable for a particularcytometry experiment. An optional light shaping element or condenserlens 403 may concentrate the emitted light at a scanning region 404,through which a cell 101 is being transported. Preferably, cell 101 hasbeen marked with one or more fluorophores that fluoresce when excited bythe light from light source 401. Many different fluorophores are known,including the ALEXA FLUOR™ series of fluorophores available from LifeTechnologies Corporation of Carlsbad, Calif., USA. The concentrationprovided by light shaping element or condenser lens 403 improves theeffective illumination of cell 101, and results in a strongerfluorescent signal. The stronger signal results in less restriction onthe exposure time of the sensor used in the system. The oblong orslit-shaped illumination field is well suited to light sources that havenaturally asymmetric illumination patterns, for example semiconductorlasers or light emitting diodes.

Light scattered from cell 101 is gathered and redirected by objectivelens 405, reflects from dichroic mirror 406, passes through tube lens408, and reaches line scan camera 409, where sequential line images ofscan region 404 are gathered for analysis by processing unit 410. Anemission filter 407 may be placed in the system to narrow the band oflight wavelengths delivered to camera 409. Dichroic mirror 406 may alsoprovide filtering. This filtering may reduce the effect of direct lightfrom source 401 that may be scattered by cell 101. Objective lens 405and tube lens 408 preferably form an infinity-corrected optical system,such that an “infinity space” is created between them. In such a system(known in the art), the performance of the system is relativelyinsensitive to the distance between the objective lens and the tubelens, allowing space for the insertion of other components such asdichroic mirror 406 and emission filter 407.

FIG. 5 illustrates an orthogonal view of a system 500 in accordance withanother embodiment of the invention. The system of FIG. 5 is configuredfor simultaneous two-color fluorescence imaging cytometry. In the systemof FIG. 5, excitation light comprising two bands of wavelengths isprovided to the cell 101 being imaged. This is represented in FIG. 5 bytwo light sources 501 producing light of different wavelengths indicatedby the solid and dashed lines. The light may be further conditioned byone or more filters 502. Other arrangements are possible. For example, asingle broad-spectrum light source may be utilized, and particular bandsof wavelengths preferentially selected by filters 502. Or a single lightsource could be used to excite two different fluorescent wavelengths. Ina preferred embodiment, light sources 501 comprise two lasers, oneproducing light in a first narrow band at a nominal wavelength of 532 nmand the other producing light in a second narrow band at a nominalwavelength of 633 nm. The light may be concentrated at the scan region504 by a light shaping element or condenser lens 503. Element 503 maycomprise various lenses, prisms, reflectors, or other opticalcomponents, singly or in combination, and preferably concentrates thelight produced by sources 501 onto an oblong area at the scan region504.

Preferably, cell 101 is marked with one or more fluorophores, such thatwhen excitation light from sources 501 reaches cell 101, light of atleast two different color characteristics is produced by fluorescence.For example, one fluorophore may react strongly to the 532 nm excitationlight, producing emitted light with an emission peak at about 550 nm,and a second fluorophore may react strongly to the 633 nm excitation,producing emitted light with an emission peak at about 650 nm. Thesedifferent emissions are approximately represented in FIG. 5 using dashedand solid lines in a way similar to the way the two colors of excitationlight are represented, although it is to be understood that lightrepresented by a particular line type after emission does not generallyhave the same spectral characteristics as excitation light representedby the same line type.

Light from scan region 504 is then gathered by objective lens 505, anddirected to dichroic mirror 506. Mirror 506 may provide some filtering,such that light principally from a band of wavelengths is reflected frommirror 506, and the remaining light passed through. The light reflectedfrom mirror 506 may pass through another emission filter 507 to furtherrestrict the spectral characteristics of the light, and then passthrough tube lens 508 and reach camera 509. Thus, camera 509preferentially receives light emitted by a first fluorophore marker incell 101, with little contamination by light from either of sources 510or from light emitted by a second fluorophore marker. That is, the lightreaching camera 509 preferably falls within a third band of wavelengthsselected from the fluorescent emissions of the first fluorophore.

The light passed through dichroic mirror 506 is then reflected fromanother dichroic mirror 510, may pass through another dichroic emissionfilter 511, passes through a second tube lens 512 and to camera 513.Thus, camera 509 preferentially receives light emitted by the secondfluorophore marker in cell 101, with little contamination by light fromeither of sources 510 or from light emitted by the first fluorophoremarker. That is, the light reaching camera 513 preferably falls within afourth band of wavelengths selected from the fluorescent emissions ofthe second fluorophore.

Cameras 509 and 513 then can scan simultaneous images of cell 101 indifferent emission spectra. The outputs of cameras 509 and 513 arepassed to processing unit 514 for storage, analysis, display, or otherpurposes. Processing unit 514 may be, for example, a computer system orother processor-based system capable of processing the image data.Processing unit 514 may be an external stand-alone device, or integratedinto a testing instrument.

Many variations are possible for the system. For example, dichroicmirror 510 may be eliminated and filter 511, tube lens 512, and camera513 positioned to directly receive the light that has passed throughdichroic mirror 506. Some of the filters in the system may be optional,depending on the particular light sources and fluorescent materialsused. Additional sets of light sources, filters, mirrors, lenses, orcameras may be added so that simultaneous imaging may be performed inthree, four, or even more different spectral bands.

One of skill in the art will recognize that the dichroic mirrors andfilters thus far described do not have perfect wavelength discriminationor perfect efficiency. Some light in the wavelength bands intended to bepassed by a particular filter may be absorbed or reflected. Some lightin wavelength bands intended to be blocked by a particular filter may bepassed or reflected. However, the filters and mirrors performsufficiently well to preferentially pass or block designated wavelengthsthat the system can discriminate different emitted light colorseffectively. In other variations, components other than dichroics may beused for color separation, including prisms, gratings, or other opticalcomponents.

FIG. 6 illustrates an orthogonal view of view of a system 600 inaccordance with another embodiment of the invention. System 600 issimilar to system 500, with the addition of slit apertures 601 and 602placed in front of cameras 509 and 513 respectively. Slit apertures 601and 602 have the effect of tending to block or exclude some lightgathered from locations other than in the focal plane of the system fromreaching the respective camera. This effect is illustrated in FIG. 6 byfinely-dashed pencil of rays 603, emanating from an out-of-focuslocation above cell 101. The resulting pencil of rays 604 emerging fromlens 508 will focus more closely to lens 508 than does the light fromthe focal plane of the system. By the time the light in pencil 604reaches slit aperture 601, pencil 604 has already started to diverge, sothat only a small portion of the center of pencil 604 can pass throughslit 601 and reach camera 509. Thus, the system preferentially receiveslight from the focal plane of the system at cell 101, and excludes atleast some light received from other depth locations.

When a small circular aperture is used in this way to limit the lightreceived by a single sensor, this technique is called confocal imaging.In the system of FIG. 6, apertures 601 and 602 are slits, and thereforeexclude light in only one axis. For the purposes of this disclosure,this is referred to as “semi-confocal” imaging. This technique improvesthe contrast of images recorded by the system as compared with imagesrecorded by a system not utilizing semi-confocal imaging.

Another advantage of a cytometry system embodying the invention is thatit may be modified or made configurable into a point-detector stylesystem, where either only a few pixels in the middle of the lineardetector are in operation or some or all the pixels in the row arebinned into one pixel or a few pixels. This results in an image ofreduced resolution in a dimension corresponding to the length of thelinear light sensor (the Y direction in FIG. 6). Each exposure of thelight sensor may even result in a single numerical representation of theamount of light falling on the sensor, for example if all of the sensorpixels are binned. Optionally, the illumination field could be shaped toa much smaller circle or ellipse, to enhance the speed of the systemwhen operating in that mode. An advantage of this kind of system is thata very high speed single cross-section image of a cell can be generated.This kind of system may be especially useful when electroniccommunication bandwidth is limited, but ample illumination is available.A system configurable in this way may be applicable to both line-scanimaging cytometry, and to non-imaging flow cytometry.

FIG. 7 illustrates an orthogonal view of view of a system 700 inaccordance with still another embodiment of the invention. In the systemof FIG. 7, simultaneous two-color fluorescence imaging cytometry isenabled using only one linear light sensor or line-scan camera. Theillumination system in system 700 may be, for example, any of theillumination systems described above with respect to system 500 shown inFIG. 5. That is, one or more light sources excites two differentfluorescence spectra, for example from two different fluorophores incell 101. Some of the light emitted by fluorescence from cell 101 iscaptured and redirected by objective lens 505 toward dichroic mirror701. The solid and dashed lines in FIG. 7 indicate that light containingtwo different fluorescence spectra reach dichroic mirror 701. Mirror 702selectively filters the light, so that one band of wavelengthspreferentially reflects from mirror 701, and other wavelengthspreferentially pass through mirror 701 and continue toward mirror 702.Additional mirrors and filters may be placed in the optical systemdirecting and conditioning the light as desired. For example, mirror 703redirects the light from mirror 702 toward tube lens 705, and mirror 703may also provide additional filtering. Similarly, mirror 704 redirectsthe light from mirror 701 toward tube lens 705, and mirror 704 may alsoprovide filtering. One or more additional filters such as emissionfilters 707 and 708 may be placed in the optical path. Tube lens 705refocuses the light onto linear light sensor 706, which may be part of aline scan camera, and can be read by processing unit 514.

The arrangement of mirrors provides a geometric offset between the twobands of light reaching sensor 706, so that part of sensor 706 receiveslight in one wavelength band, selected from the light emitted in one ofthe fluorescence spectra, and another part of sensor 706 receives lightin the other wavelength band, selected from light emitted in the otherfluorescence spectrum. For example, if sensor 706 comprises 512 pixelsarranged in a row, then approximately the first 256 pixels may receivelight in one band of wavelengths, while approximately the remaining 256pixels may receive light in the other wavelength band. As above,processing unit 514 receives repeated line scans from sensor 706, andcan reconstruct two images of cell 101, one image for each wavelengthband. Such a system requires only one linear light sensor or line scancamera, and may be constructed at reduced cost as compared with a systemhaving two linear light sensors or line scan cameras. Other kinds ofoptical systems may also be used to direct light in two wavelength bandsto separate portions of a linear light sensor. For example, such anoptical system may comprise an optical grating. A slit aperture may beincluded in a system such as system 700, so that the system performssemi-confocal imaging.

FIG. 8 illustrates an orthogonal view of view of a system 800 inaccordance with still another embodiment. System 800 is illustrated as avariant of system 400, shown in FIG. 4, but one of skill in the art willrecognize that the additional features of system 800 may be employed inother systems, including ones that perform multi-color imaging,fluorescence imaging, or other techniques.

System 800 employs an exemplary camera 801 having three closely spacedparallel rows of sensors 802, 803, 804. (The sensor rows are shownend-on in FIG. 8.) By virtue of the operation of the optics of thesystem, each of the rows images a different “stripe” on cell 101. Camera801 thus has three different opportunities to image any particular partof cell 101 as cell 101 passes by the scanning region 404. That is, aparticular part of cell 101 will be imaged onto row 802 at a first time.That same part of cell 101 will be imaged onto row 803 at a later time,and onto row 804 at a still later time. In FIG. 8, only the central raysof pencils connecting cell 101 with sensor rows 802, 803, 804 are shown,so as not to obscure the operation of the system in unnecessary detail.

In one technique, three different images may be gathered of cell 101,one made by each of sensor rows 802, 803, 804. The different images areshifted in time with respect to each other, or may also be thought of asshifted in space, in the X direction. These multiple images may be usedto create a composite image with improved signal-to-noisecharacteristics. For example, if the three images are digitally shiftedback into alignment and pixel values from the three images correspondingto substantially the same locations on cell 101 added, the resultingcomposite image will have a signal-to-noise ratio improved by a factorof approximately √{square root over (3)} as compared with any one of theindividual images. While camera 801 has been illustrated as having threescan lines, it may have 2, 4, or any usable number n. A composite imageproduced by this digital addition or averaging technique from a camerahaving n lines will have a signal-to-noise ratio improved by a factor ofapproximately √{square root over (n)} as compared with a single image.The combination of the images may be done “on the fly” as the scannedimage lines are available, so that no complete image of a particularcell made by a single linear sensor is constructed.

Camera 801, having multiple rows of pixels, may additionally oralternatively be configured to perform time delay integration (TDI). InTDI, the electrical charges in the various pixels resulting from anexposure to cell 101 are accumulated within the pixel rows beforeconversion to digital values. The exposures of the sensors to cell 101are substantially synchronized such that a particular location on cell101 is exposed to sensor row 802 during one exposure, to sensor row 803during the next exposure, and to sensor row 804 during the nextexposure. Charges accumulated in row 802 during the first exposure areshifted into row 803 and added to by the second exposure, and theresulting charges are shifted into row 804 and added to by the thirdexposure. The accumulated charges are then converted to digital values.TDI also results in an approximately √{square root over (n)} improvementin signal-to-noise ratio as compared with a single image.

One advantage of scanning simultaneous parallel image lines, whether foruse with digital image combination or TDI, is that the technique takesbetter advantage of the available illumination. A light shaping elementsuch as element 403 will not generally focus light onto asingle-pixel-wide strip at the scan line. The illumination field willhave some appreciable width, and some of the illumination may be wastedin a single-line camera system.

Another advantage of such a system is that the resolution is notcompromised, as it may be in systems that simply bin pixels in order toimprove signal-to-noise characteristics.

One of skill in the art will recognize that a system such as system 500shown in FIG. 5 could also be adapted such that each camera 509, 513includes a set of two or more linear light sensors. Imaging would beperformed by each camera 509, 513 as described above with respect tocamera 801, so that multi-color imaging may be accomplished by digitalimage combination or TDI.

Similarly, a system such as system 700 shown in FIG. 7 could be adaptedso that sensor 706 is replaced by a set of at least two linear lightsensors. The system would then direct wavelength-selected lightseparately to two portions of the set of linear light sensors.

The systems of FIGS. 2, 4, 5, 6, and 7 may be thought of as including a“set” having a single linear light sensor.

Additionally, combining images from at least two parallel linear lightsensors, whether by digital combination or by time delay integration,can be combined with binning or other resolution-reducing techniques.Binning may produce an image with further improved signal-to-noisecharacteristics, albeit at a reduced resolution.

FIGS. 9A-9C illustrate additional techniques for providing an oblongillumination field convenient for performing line-scan cytometry.

The line-scan cytometry technique may not require the use of an oblongillumination field in all embodiments. Conventional circularepi-illumination may be utilized, providing the illumination power issufficiently high. For imaging using scattered, non-fluorescent light,sufficient power of the illumination source may not be difficult toachieve. However, for practical sensing of light emitted byfluorescence, concentrating the excitation light into an oblong fieldcan be much more energy-efficient, for example reducing the requiredexcitation laser power from a level measured in tens or hundreds ofwatts to a level measured in tens or hundreds of milliwatts.

FIG. 9A illustrates one view of a system 900 that includes an embodimentof a technique for providing an oblong illumination field. System 900uses some components and arrangements similar to those of system 400shown in FIG. 4, but one of skill in the art will recognize that theillumination technique illustrated in FIG. 9A may be used with othersensing arrangements as well. In system 900, illumination is provided bya laser 901 from the same direction as from which sensing is performed,so that the space above the sample is left unobstructed. Thisarrangement may therefore accommodate much larger samples that thearrangements previously described, which may be limited to samples nothicker than the distance between the sample stage and the condenserlens. Another advantage of the system of FIG. 9A is that objective lens405 participates in the formation of the illumination field. Objectivelens 405 may typically be a very high-quality lens, so that theillumination field it produces may be very sharply defined.

In the example system of FIG. 9A, laser 901 produces a beam 902,directed at cell 101. Beam 902 passes through a cylindrical lens 903.For the purposes of this disclosure, a cylindrical lens is any lens thathas curvature in only one dimension. A cylindrical lens may but need nothave curved surfaces defined by circular cylinders. In the view of FIG.9A, cylindrical lens 903 is positioned with its cylindrical axisparallel to the X direction, and lens 903 appears to have no effect onbeam 902. Beam 902 continues through dichroic mirror 406 to objectivelens 405, which focuses the beam onto cell 101. Light emanated from cell101 passes through objective lens 405, preferentially reflects frommirror 406, may encounter one or more filters 407, passes through lens408, and reaches camera 409.

FIG. 9B illustrates an embodiment of the illumination portion of FIG.9A, from a view along the X axis. That is, FIG. 9B shows a view rotated90 degrees from the view of FIG. 9A. In this view, tube 102 projects asa circle, and cylindrical lens 903 shows as having a curved profile. Inthe example embodiment of FIG. 9B, the materials and dimensions ofcylindrical lens 903 are selected such that lens 903 has a relativelylong focal length—greater than the distance between the cylindrical lensand the objective lens. After passing through cylindrical lens 903, beam902 is seen to relatively gradually converge, as seen in this view.Objective lens 405 then converges and reexpands the beam in the Ydirection, such that the illumination field is widened. As is shown inFIG. 9A, objective lens 405 simultaneously focuses the beam in the Xdirection. The resulting illumination field may have a sharply-definedoblong shape as it encounters cell 101.

FIG. 9C illustrates another embodiment of the illumination portion ofFIG. 9A, from a view along the X axis. In this embodiment, the materialsand dimensions of cylindrical lens 903 are selected such that lens 903has a relatively short focal length—shorter than the distance betweenthe cylindrical lens and the objective lens. After passing throughcylindrical lens 903, beam 902 is seen to converge and then redivergebefore reaching objective lens 405. Objective lens 405 redirects beam902 such that it again converges, but slowly enough that when beam 902reaches cell 101, beam 902 is still sufficiently wide to span at least aportion of the line being scanned by camera 409. Again, objective lens405 simultaneously focuses the beam in the X direction. The resultingillumination field may have a sharply-defined oblong shape as itencounters cell 101.

While embodiments of the invention have been illustrated as scanningcells confined in a linear tube, one of skill in the art will recognizethat embodiments of the invention may be utilized in systems using anyof a wide range of cell delivery techniques, including electrophoresis,pressure driven flow, optical tweezers, motorized translation stage, andothers. Cells may be conveyed as a payload in an oil emulsion, in anelectrowetting-actuated droplet, or via magnetic transport assisted bymagnetic bead tagging. It is intended that the claims not be limited bythe cell delivery method utilized.

In the claims appended hereto, the term “a” or “an” is intended to mean“one or more.” The term “comprise” and variations thereof such as“comprises” and “comprising,” when preceding the recitation of a step oran element, are intended to mean that the addition of further steps orelements is optional and not excluded. The invention has now beendescribed in detail for the purposes of clarity and understanding.However, those skilled in the art will appreciate that certain changesand modifications may be practiced within the scope of the appendedclaims.

What is claimed is:
 1. A system for performing cytometry, the systemcomprising: a scanning region that is illuminated by light including atleast first and second wavelength bands; a cell transport mechanismtransporting a cell through the scanning region such that the cell isilluminated; a set comprising at least one linear light sensor; and anoptical system that selectively directs light emitted from the cell totwo portions of the linear light sensor set such that emitted light in athird wavelength band is primarily directed to a first portion of thelinear light sensor set, and emitted light in a fourth wavelength bandis primarily directed to a second portion of the linear light sensorset; wherein the system repeatedly takes readings of light falling onthe linear light sensor set while the cell is transported through thescanning region.
 2. The system of claim 1, wherein the set comprises atleast two linear light sensors.
 3. The system of claim 1, wherein theemitted light is emitted as a result of fluorescence.
 4. The system ofclaim 1, further comprising a slit aperture proximate the linear lightsensor set, such that the system performs semi-confocal imaging.
 5. Thesystem of claim 1, wherein the set comprises at least two linear lightsensors, and wherein images gathered by the individual linear lightsensors in the set are combined to form an image with improvedsignal-to-noise characteristics as compared with an image gathered by asingle linear light sensor in the set.
 6. The system of claim 5, whereinthe images are combined by digitally combining pixel values from therespective images corresponding to substantially the same respectivelocations on the cell.
 7. The system of claim 5, wherein the images arecombined by time delay integration.
 8. A system for producing an oblongillumination field, the system comprising: a laser that produces a beam;an illumination lens that receives the beam and causes the beam toconverge or diverge in only a first axis; an objective lens that is partof an infinity-corrected optical system, the objective lens receivingthe beam after the illumination lens and converging the beam in a secondaxis orthogonal to the first.
 9. The system of claim 8, wherein theillumination lens is a cylindrical lens.
 10. The system of claim 9,wherein the illumination lens has at least one curved surface defined bya circular cylinder.
 11. The system of claim 8, further comprising awavelength-selective mirror between the illumination lens and theobjective lens.
 12. The system of claim 8, wherein the objective lens isspaced from the illumination lens by a distance less than the focallength of the illumination lens.
 13. The system of claim 8, wherein theobjective lens is spaced from the illumination lens by a distancegreater than the focal length of the illumination lens.
 14. The systemof claim 8, wherein the beam is diverging in the first axis as it leavesthe objective lens.
 15. The system of claim 8, wherein the beam isconverging in the first axis as it leaves the objective lens.